First post, so be gentle with me!
I love my 160GB iPod classic, as it's the only player out there that holds all my music. I actually think that the headphone amp on the classic sounds pretty good - my headphones are Shure E500s and B&W P5s, and through both of those, the bog-standard classic sounds great to me. The previous generation did sound a little better in terms of tone, but that was offset by a level of hiss that I found unacceptable.
Anyway, I've got fed up with the number of times I've read people online claiming that Cowons etc all sound orders of magnitude better than the iPod. I was sceptical, but I was keeping an open mind, and did wonder if my iPod could actually sound better. I listen to it a lot at work, and have toyed with various external amp options in order to get the best from it, but as soon as I saw the HP-P1 was being introduced, I decided I had to have one. I was onto the UK distributor about 20 minutes later, and I suspect I may have had the first one in the UK - I've had it about a month.
So - the verdict? Well, bear in mind this is the first dedicated headphone amp I've tried, but I have been using headphones that cost more than the iPod does for several years, and my home system is high-end Arcam/B&W. Music on my iPod is in high bit-rate VBR AAC - about 200 kb/s, as to me that is past the threshold of audibility. I listen to mostly rock and acoustic music.
With the B&W P5s, the HP-P1 makes, as far as I can tell, no difference whatsoever compared to the iPod's own headphone socket. If there is an improvement in the sound, it is so tiny as to be imperceptible. This was a bit of a disappointment, so I tried my Shures...
With the Shures, the P1 does make a difference - the sound gets a bit less warm in the upper midband, which has the effect of making different instruments easier to separate and giving a small improvement in clarity. It isn't a huge effect, but it is noticeable. That said, in some ways I actually prefer the sound of the iPod's headphone out - the P1 sounds a bit light in the bass by comparison, and I think I prefer the slightly more bassy sound of the iPod itself.
All in all - quite disappointing for the money. (I paid £500 for mine.) Also, there is a silly design flaw with the P1 - even when plugged into a USB power source, it doesn't charge the iPod via the USB connection, so you can't just leave the iPod and P1 combo plugged into a mains adaptor for use at your desk - you still have to keep unplugging the iPod to charge it.
No complaints about the construction or build quality, although the supplied carrying case seems designed to fit a much bigger player than even the largest iPod, and is pretty badly thought out.
I'm probably going to sell mine, to be honest - it just doesn't make enough of a difference to be worth the rather high asking price.
Happy to answer questions if anyone has them - I feel like a bit of a guinea pig on this product to be honest!